British Pat. No. 2167744 describes a series of gamma lactones having the general formula: ##STR2## in which Ad.dbd.C&lt; represents an adamantylidene group, R.sub.1 is selected from hydrogen, alkyl and aryl and ##STR3## represents a carboxylic or heterocyclic group, such as furyl or thienyl. The utility of the gamma lactones described in the above patent specification is as anti-tumoural compounds or as intermediates for producing compounds having such pharmacological activity.
We have now discovered that structurally related lactones have valuable photochromic properties.